Some Linux distributions (I've observed this on Debian) restrict
unprivileged users' ability to create namespaces, so checking just
whether the "unshare" command is available is not enough: If it is but
the user building mod_gnutls does not have the necessary permissions,
test cases will fail when trying to create their namespaces. Add a
check that tries to actually create a namespace and disable namespace
isolation of tests if it fails.